An apparatus for producing foamed or expanded thermoplastics materials is disclosed in German Patentschrift No. 1 135 652. Such apparatus comprises a fusing and homogenising extrusion device connected in series with a cooling device. The thermoplastics material, which has previously been mixed with a propellant, is treated in the fusing and homogenising extrusion device and is then conveyed into a static cooling device through a long conduit.
Because of the conveying pressure of the fusing and homogenising extrusion device, the material to be cooled must be extruded through the conduit, which latter has a relatively small diameter, and through the cooling unit. In addition, a tool such as a blow head is usually provided at the outlet end of the cooling unit. This tool sets up a back pressure which must also be overcome if the material is to continue its travel through the apparatus.
The homogenised material, which has been mixed with propellant, therefore needs to be subjected to a high pressure to enable it to progress. This, however, has the disadvantage of causing energy to be released which causes the thermoplastic material to become heated. In consequence, most of the cooling power of the cooling device, which is located at the downstream end of the apparatus is needed to remove the heat introduced into the material by friction. Accordingly, the intended purpose of the cooling device, which is to cool the mixture of material and propellant to a temperature below the decomposition temperature of the propellant, is not adequately achieved. Moreover, such cooling is particularly uneconomical.
Any changes in the direction of flow of the mixture of material and propellant also causes a certain amount of frictional heat to be introduced into the mixture. Such heat must be removed before the mixture emerges from the extrusion tool. This can only be achieved with great difficulty and by utilising further, more powerful, cooling devices.